1. Field of the Invention
The present invention relates to a method of manufacturing a vertical-cavity surface-emitting semiconductor laser element, and more specifically to a method of manufacturing a vertical-cavity surface emitting semiconductor laser element as a type of narrowing an oxidized layer, that comprises a mesa post in which a structure of a resonator in a vertical direction is designed to be formed in a direction as vertical to a substrate, and that is superior in a reliability.
2. Description of the Related Art
A vertical-cavity surface-emitting semiconductor laser element (VCSEL: referred to as a surface emitting laser element hereinafter) is a semiconductor laser element that is designed to emit a light in a direction as orthogonal to a substrate. Moreover, in accordance with such the surface emitting laser element, it becomes possible to array laser elements as plurality thereof in a two dimensional array formation on to a same substrate therefor. Further, an attention is becoming to be paid to such the surface emitting laser element as a source of light for a usage of a communication, or as a device for a usage of any other applications as a variety thereof. Furthermore, a demand of such the surface emitting laser element is becoming to be increased mainly for a usage of a source of a signal for a high speed optical transmission in particular regarding a data com transmission such as a giga bit Ethernet™ or a fiber channel or the like.
Here, FIG. 5 is a cross sectional view for showing a configuration of a conventional vertical-cavity surface-emitting semiconductor laser element that is disclosed in the following Patent Document 1. And, such a vertical-cavity surface-emitting semiconductor laser element (10A) comprises: a laser structure in which distributed Bragg reflector (DBR) mirrors of (14) and (22) as one pair thereof are designed to be formed, that is formed of a multilayered film layer of a semiconductor on to a substrate of a semiconductor (12), such as a GaAs or an InP or the like; and an active layer (18) and a cladding layer at a lower side thereof (16) and a cladding layer at an upper side thereof (20) that are designed to be as a region for emission between such the DBR mirrors of (14) and the (22) as the one pair thereof. Moreover, such the element (10A) further comprises an electrical current narrowing layer (24) (a narrowing layer by oxidizing) in order to enhance an efficiency of an electrical current through such the active layer (18) and then in order to reduce a threshold electrical current for an oscillation. Further, the electrical current narrowing layer (24) is designed to be comprised of a layer of an AlAs, and then the same comprises: an inhibition region from an electrical current (24B) for which a periphery of such the layer of the AlAs is designed to be oxidized; and a region for injecting an electrical current (an open part for the electrical current) (24A) that is designed to be remained at a central part as a region which is not designed to be oxidized. Still further, the DBR mirrors of (14) and the (22) is designed to be made use of a plurality of pairs of layers, such as a layered structure of Al(Ga)As/AlGaAs or the like as a system of a GaAs for example.
Still further, it is able to design for such the surface emitting semiconductor laser element of the vertical cavity type (10A) to be formed on to a substrate of a GaAs. Still further, it is able to design to make use of a DBR mirror that is formed of a system of an AlGaAs which is superior in a thermal conductivity thereof and has a ratio of reflectance as higher. And then such the element (10A) becomes to have a good prospect as a laser element by which becoming able to emit a laser light of a band between 0.8 μm and 1.0 μm.
Furthermore, such a surface emitting semiconductor laser element becomes to have a good prospect as a laser element, in which the active layer (18) is designed to be formed with making use of a material of a GaInNAs system, thereby becoming able to emit a laser light of a band in a long wave length that is between 1.2 μm and 1.6 μm.
Here, it is known that a volume of the layer of the AlAs becomes to be retracted at a period of performing a process of forming the electrical current narrowing layer (24) by performing an oxidation of the layer of the AlAs in accordance with the vertical-cavity surface emitting semiconductor laser element as the type of narrowing the oxidized layer which is described above, and then that a stress in the layer of the compound semiconductor becomes to be occurred which is adjacent to such the electrical current narrowing layer (24). And then due to such the stress to be occurred, because the active layer (18) is designed to be located in a vicinity of such the electrical current narrowing layer (24), it becomes to be occurred the following problems that a damage becomes to be occurred on to such the active layer (18), and then that a life time of such the element becomes to be shorter. And therefore making use of a layer of an Al0.98Ga0.02As is proposed, that includes gallium (Ga) with a small amount thereof in place of the layer of the AlAs for such the electrical current narrowing layer (24) in order to prevent from shortening the life time of such the element. Moreover, an effort is given to reduce such the stress that is generated due to the retraction of the volume at a time of performing a conversion into such the electrical current narrowing layer (24), by designing for such the layer of the AlAs to have a thickness of approximately 40 nm as thinner.
In the meantime, the multilayered film layer that configures the DBR mirror becomes to be exposed as well to a condition of oxidation as stronger which is similar to that for such the layer of the AlAs. And hence it is occurred a problem that it cannot help but be oxidized a layer of an Al0.9Ga0.1As to become as an annular shape along the periphery of the mesa post, that is shown with making use of the symbol (27) in FIG. 5, that is designed to be as a layer in which a composition of Al is higher, that is to say, that is to be as the layer having the index of refraction as lower among the pairs of the layers that configure the DBR mirror at the upper side (22).
Moreover, such the oxidized part (27) that becomes to be formed due to an oxidation at an inner side of the DBR mirror has a width of approximately a few hundreds of nanometers for example, that of course further depends on such as a composition or a thickness or the like as well regarding a layer of a compound semiconductor which configures such the DBR mirror. And then for each of the layers of the AlGaAs that individually configure such the DBR mirror a designing is performed to have a layer thickness to be λ/4n as relatively thicker with corresponding to a wave length of the laser light that is defined here to be as the λ, though such a degree of the oxidation as a few hundreds of nanometers is relatively smaller from a point of view of a diameter of a mesa post (38). Further, such the DBR mirror is designed to have the number of pairs as large in addition to such the layer thickness of the DBR mirror as relatively thicker. And hence the retraction of the volume of the oxidized part becomes to be so much larger that it cannot be negligible comparing to the other part at an inner side of such the surface emitting semiconductor laser element of the vertical cavity type even that has the width of just a few hundreds of nanometers. And therefore a development of such as a surface emitting semiconductor laser element of the vertical cavity type or the like is performed as well, that is designed to comprise a DBR mirror in which a layer having an index of refraction as lower that is a layer containing Al as higher is designed to have a ratio of composition of Al with being suppressed within a predetermined ratio, in order to prevent the inner side of such the DBR mirror from any occurrence of the oxidation.    [Patent Document 1] Japanese Patent Application Publication No. 2007-258582    [Nonpatent Document 1] C. Helms, I. Aeby, W. Luo, R. W. Herrick, A. Yuen, “Reliability of Oxide VCSELs at Emcore”, Proc. SPIE, vol. 5364, pp. 183-189, 2004
Moreover, a problem on a reliability of a random failure (an unexpected sudden failure) is inhered in such a vertical-cavity surface emitting semiconductor laser element as the type of narrowing the oxidized layer. And then it is known that a cause is mainly a stress due to an oxidation of a part that is not intended at the period of the process of the selective oxidation that is described above. Moreover, it can be considered for such the cause that a variation of the volume becomes to be occurred in accordance with such the surface emitting semiconductor laser element of the vertical cavity type due to the cause of the oxidation of the layer having the index of refraction as lower in the DBR mirror, that a stress which is generated due to such the variation of the volume becomes to effect as negatively on to the active layer, and then that a dislocation becomes to be generated on an end face of such the active layer at a lateral face of the mesa post. And then in accordance with such the surface emitting semiconductor laser element of the vertical cavity type, such the dislocations that are generated at a period of performing the manufacturing becomes to be increased even to the region for emitting at the central part of the mesa post. And hence such the region for emitting becomes to include defects, and then thereby resulting in to the random failure.
Here, it can be mentioned that it may be effective by making use of a layer of a semiconductor in which a layer having an index of refraction as lower is designed to have an Al content with being suppressed in order to suppress such the occurrence of the oxidation of the DBR mirror. However, the index of refraction of the layer containing Al regarding a semiconductor layer of the AlGaAs system is different with corresponding to a ratio of an Al composition. And thus there is a demand as larger in order to design such a difference of the index of refraction between a layer having an index of refraction as higher and the layer having the index of refraction as lower by designing to change the ratio of the component of Al respectively, and then in order to form a DBR mirror that is designed to have a ratio of reflectance as higher. In the meantime, however, it is not able to enhance such the difference of the index of refraction between the layer having the index of refraction as higher and the layer having the index of refraction as lower in a case where the content of Al in such the layer of the AlGaAs is designed to be suppressed as lower. And then thereby becoming indispensable to design for the number of the pairs of the layered film layers to be increased in order to form a reflecting mirror that is designed to have a predetermined ratio of reflection. In the meantime, however, it is not desirable to increase such the number of the pairs of the layered film layers as too many from a point of view of such as a property of a heat radiation or a property of a light transmission or a manufacturing cost or the like.
Moreover, in accordance with such the surface emitting semiconductor laser element of the vertical cavity type that is disclosed in the above Patent Document 1, it is disclosed that a layer of a compound semiconductor that has a content of Al as lower is designed to be formed at a part for forming a layered film layer in a vicinity of an active layer therein among the layered film layers that individually configure a DBR mirror, in the meantime, that another layer of a compound semiconductor that has a content of Al as higher is designed to be formed at a part for forming a layered film layer which is far from such the active layer on the contrary. And then according to such the configuration in such a manner, it becomes able to reduce an amount of the distortions that become to be applied from such the part for forming the layered film layer in the vicinity of the active layer to such the active layer. Further, it becomes able to reduce the number of the total layers regarding such the layered film layer that is designed to configure such the DBR mirror, and then it becomes able to suppress the increase in the number of the pairs of such the layered film layer by adopting the layer of the compound semiconductor that has the content of Al as higher, in order to form such the layered film layer that has the stress due to the distortion as relatively smaller which might effect as negatively on to the active layer and that is designed to be located as far from such the active layer. However, an advantage is obtained as limiting somehow by making use of such the method that is disclosed in such the above Patent Document 1.
In the meantime, a configuration is disclosed in the above Nonpatent Document 1 in order to remove a peripheral part of a mesa post for an oxidized layer having an index of refraction as lower. However, there is no disclosure at all regarding any process of removing such the part as more specifically to a method in such the document.
And therefore the present invention is provided with having a regard to the description as above, and an objective is to provide a method of manufacturing a vertical-cavity surface-emitting semiconductor laser element which is characterized in that it becomes able to perform a process of removing as effectively a part that will be oxidized in a DBR mirror in order to suppress a stress that effect as negatively on to an active layer due to a cause of any occurrence of the oxidation in such the DBR mirror at a period of manufacturing such a surface emitting semiconductor laser element that comprises a mesa post.